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September 2017 • SMT Magazine 53 REWORK AND REBALL CHALLENGES FOR WAFER-LEVEL PACKAGES and further FA. Due to their small form factor and fragility, WLPs can prove particularly diffi- cult for the rework and reball process steps. The present work provides an overview of the FA process flow, emphasizing component rework and reball methodologies. The challenges of a specific WLP case study are discussed, and re- work and reball improvements are implement- ed to minimize thermally- or mechanically-in- duced artifacts. Fault isolation is an important first step in the system-level FA process. By isolating the fail- ure to a specific package or interconnect, an op- timal FA approach can be assessed and through- put time can be greatly reduced. Several fault isolation techniques can be used to measure for opens and shorts, including hand probing, time domain reflectometry and the newer electro op- tic terahertz pulse reflectometry [2]. High-resis- tance shorts can also be detected by powering the board and measuring the localized temper- ature increase using infrared thermal imaging techniques [3]. After the failing component has been iso- lated, nondestructive FA is performed to in- spect for gross board- or package-level failures. OM can be used to inspect for external board or package defects, such as foreign materials or su- perficial die and overmold cracks. Since WLPs have exposed dielectric layers (and since fan- in WLPs have exposed bulk Si), it is especial- ly important to perform a first pass inspection for external chips or cracks. It is recommended that optical inspection be performed again after rework and reball, to confirm that no artifacts were introduced. CSAM, a popular non-destructive tech- nique, can be utilized to detect internal defects or to evaluate the extent of external damage. CSAM uses an ultrasound transducer to raster- scan the package backside. At material interfac- es, an acoustic pulse is reflected to the transduc- er and recorded as signal amplitude. Air-solid interfaces occur at the locations of cracks, voids, or delamination, and return high-intensity re- flections. CSAM is thus a valuable metrology for identifying gross internal package defects that cannot be detected with simple optical inspec- tion. CSAM can also be used to screen for re- work and reball artifacts, particularly thermal- ly-induced delamination. 2D X-ray is another common technique that provides an effective "quick pass" inspec- tion for board-level solder defects, such as voids and bridging. By optimizing the sample tilt and rotation angles, subtler non-wet open and non- contact open defects can also be detected. How- ever, 2D X-ray is not capable of detecting sub- micron defects, such as board-level solder or via cracks. Recently, more advanced imaging metrolo- gies have emerged as powerful non-destructive FA techniques. In particular, 3D X-ray comput- ed tomography has proven effective at detect- ing both board-level and package-level sub-mi- cron defects [2,4] . Multiple 2D X-ray images are collected as the sample is rotated at fixed an- gle increments. The 2D X-ray images are then superimposed to generate a three-dimensional volume. The superimposed image can be ma- nipulated to display virtual "slices" of the sam- ple, allowing for inspection of the solder joints, via barrels, and traces. If board-level or gross package-level failures are not detected using non-destructive tech- niques, the package must be sent for socketed component-level testing and further FA. Stan- dard test sockets use a spring-loaded floating base that is guided by the solder balls instead Figure 2: Diagram showing the typical process flow for system-level failure analysis.